A known switching power source device comprises a primary winding of a transformer and a switching element connected in series to a DC power source wherein switching element is turned on and off to cause electric current to flow through primary winding and switching element in order to generate DC power from output terminals connected to a secondary winding of transformer. When switching element is turned on, electric current passes through primary winding of transformer to accumulate electro-magnetic energy in transformer, and when switching element is turned off, stored electro-magnetic energy can be taken out from secondary winding in a switching power source device named “Flyback Converter”. However, the converter has a drawback of lowered efficiency in power conversion because a rectifying diode connected to secondary winding of transformer incurs power loss due to voltage drop in the forward direction upon the conduction of the diode.
Then, to reduce power loss resulted from voltage drop in the forward direction of the diode connected to secondary winding, a switching power source device has been proposed in the following Patent Document 1 wherein a power MOS-FET as a switching element in a synchronous rectification circuit is connected to secondary winding to prevent passage of adverse current and reduce voltage drop in the forward direction, thereby mitigating operational burden of transformer. This switching power source comprises a pulse transformer which includes a main converter circuit such as a flyback or forward converter circuit; a power MOS-FET of synchronous rectification connected to a secondary winding of pulse transformer for rectifying output current from secondary winding; and a guard comparator for monitoring change in direction of electric current flowing between source and drain terminals of power MOS-FET of synchronous rectification to control the forward and backward currents by a gate drive signal supplied from guard comparator to a gate terminal of power MOS-FET of synchronous rectification.
Patent Document 1: Japanese Patent Disclosure No. 7-7928 (Page 6, FIG. 1)